This invention relates to a radar device for projecting a near-infrared laser beam forward and measuring the distance to an object by observing its reflected light, as well as an MSMS mirror device that is appropriate therefor.
Radar devices for projecting a near-infrared laser beam forward to monitor the presence of objects such as a front going vehicle, pedestrians and obstacles and to measure their distances and directions by observing its reflected light are coming to be used popularly in recent years. Conventionally, a PIN-type photodiode was commonly used as the light receiving element for receiving the reflected light. Since PIN photodiodes have a low S/N ratio and the sensitivity cannot be made high, there was a limit to the detection of reflected light with low brightness being returned from a distant or black object.
For this reason, Japanese Patent Publication Tokkai 11-160432, for example, disclosed a radar device using a photodiode called avalanche photodiode (hereinafter referred to as APD), which has a high S/N ratio and high sensitivity and is capable of detecting reflected light with low brightness. An APD is usually inserted into a reverse bias circuit and can operate with high sensitivity if the bias voltage is increased.
Japanese Patent Publication Tokkai 11-288444, on the other hand, proposed a bar code scanner for using projected light from a laser diode and reading reflected light from a target object, providing integrally a mobile part of its scanning means with reflecting means and light detecting means by a semiconductor process.
Objects may be present in front of an automobile at different distances from very near to very far and they are widely different in reflectivity. Thus, if a radar device is to serve as a monitor for the forward direction, its dynamic range for the reflected light to be monitored is required to be extremely wide.
In general, a photodiode becomes saturated by an input with high brightness and is not capable of detecting an input with low brightness below the noise level of the element. Thus, if its light receiving sensitivity is increased, incident light with low brightness may become detectable but it becomes saturated with input light with high brightness. If the sensitivity is reduced such that incident light with high brightness can be observed, on the other hand, incident light with low brightness sinks below the noise level.
It is therefore an object of this invention to provide a radar device capable of accurately measuring the position of an object even if reflected light has a wide dynamic range and an MEMS mirror device that can be applied to such a radar device.